This disclosure relates to imaging systems and, in particular, to imaging systems for transferring marking material using a latent resistive layer.
Printing technologies fall into two distinct groups: those that are digital and allow every printed page to contain variable text and images and those that are master plate based and allow high volume duplication of a single image. Common examples of digital printing technologies include inkjet, electrophotography (EP), and thermal transfer. Common examples of master based duplications technologies include offset lithography, flexography, and gravure.
Unfortunately, all of the digital printing technologies are severely limited in speed as compared to the master based duplication processes. This speed limitation reduces their productivity and fundamentally limits their economics to copy run lengths no larger than a few hundred copies. In the case of inkjet printing, the marking inks consist of very dilute pigments or dyes in a solvent carrier and print speed is limited by the energy require for solvent evaporation. In the case of electro-photography, print speed is limited by the energy required for toner fusion. Finally, the print speed for thermal transfer is limited by the energy that is required to transform inked material on a ribbon from either a solid into a liquid or for the case of dye diffusion thermal transfer (D2T2), the energy from a solid to a gas. A large amount of energy is required for these thermal methods because the ink must be raised above a phase change temperature and the latent heat of melting or evaporation must be delivered. In addition to these considerations, the lower pigment concentration of typical digital marking materials can lead to higher marking pile height or image bleed. This is undesirable in terms of gloss uniformity, tactile feel, stacking thickness for books, and fold fastness. Furthermore, each of the digital marking materials usually has a much stricter limitation on color gamut and substrate latitude and size when compared with offset lithography.
In waterless offset technologies, a patterned polydimethylsiloxane (PDMS) layer, commonly referred to as silicone, is used to block the transfer of ink. That is, silicone is used to prevent the transfer of the ink. Under the rapid shearing forces of the NIP, the viscoelastic cohesive forces within the ink can exceed the surface adhesion force at the silicone interface and the ink is rejected from the non-image areas of the cylinder. In non-silicone regions the adhesive forces overcome the built-in cohesive forces of the ink and the ink film splits apart thus leaving behind a layer of ink in the imaging areas.
In most offset printing systems, the mass ratio of ink film splitting in these imaging areas such as between the imaging plate and the offset blanket is usually a faction between 30/70 and 50/50. In practical terms, this means that roughly 10 blank pages are needed to remove enough ink so that the previous image is no longer visible. This is not a problem when running long jobs because much of the make ready paper is used to tune the color and alignment of colors on a page so no additional cost is of concern. This is an issue when variable data is introduced because ghosting can result from the remaining ink from a prior image.
There have only been a few attempts at high quality high speed variable data digital printing with higher pigment concentration inks. Gravure and flexography inks with viscosities in the range of 50-1000 cp have been shown to respond to electrostatic pulling over short distances. However, the electrostatic forces are too weak to work with high viscosity high pigment concentration offset inks with viscosities above 100,000 cps.
Currently, these issues make it incredibly challenging to print highly viscoelastic marking materials such as offset or waterless offset inks (i.e. marking materials having dynamic viscosities of 10,000-1,000,000 cps) in a digital fashion with variable data on each and every page.